<p>Rapid industrial progress in the modern era poses significant threats to both the environment and human health, as many industrial processes involve the use of toxic gases and produce hazardous byproducts. Phosphine (PH₃), being poisonous and flammable, is difficult to detect owing to its colourless and odorless nature, posing serious risks to both ecological systems and human well-being. To mitigate the risks linked to PH₃ exposure, two-dimensional (2D) Indium Selenide (InSe) with hexagonal geometry has been proposed for its detection based on first-principles calculations. The effects of PH₃ adsorption on the structural, electronic, interfacial, and gas sensing properties of 2D InSe are systematically calculated. Notably, the adsorption of PH₃ gas alters the band gap (E_g) of InSe monolayer. The band structure analysis reveals that E_g value (1.467&#xa0;eV) belonging to the pristine InSe decreases to 1.432, 1.451, 1.457 and 1.443&#xa0;eV upon PH₃ adsorption at the hollow (H), top chalcogen (TC), bridge (B), and top-metal (TM) sites, respectively. A deeper examination of the interfacial interaction reveals the depletion of charges for H, TC, and TM sites, whereas charge accumulation is observed for B site near InSe monolayer. The findings suggest that H site exhibited the highest increase in conductivity ratio, making it the most prominent, with 52%, 48%, and 24% greater sensitivity to the PH₃ gas molecule compared to the B, TC, and TM adsorption sites, respectively. Aforementioned attributes highlight 2D InSe as an apposite candidate for developing toxic recovery-type gas sensing device.</p>